FISH. 



353 



bony plate, the operculum (fig. 198, o). The limbs when present 

 are represented by fins ; and the heart, except in one group, the 

 Dipnoi, consists of two chambers only, a single auricle and a ven- 

 tricle. The blood runs back from the body into the auricle (a), 

 and from thence through the ventricle 

 it is sent to the gills. (&) to be purified. 

 From the gills it runs on as arterial 

 blood to the various parts of the body. 

 The blood is only pumped to the gills ; 

 tlms the heart is a purely respiratory 

 one. A skeleton is always present, 

 either bony or cartilaginous. In the 

 lowest fishes — Sharks, Eays, &o. — it 

 is cartilaginous ; in the more highly 

 developed fish — Teleostei — it is bony. 

 The sexes are always distinct, ova or 

 spawn being deposited by the females. 

 The young fish or " fry " are like the 

 parent ; but a few, such as the Lam- 

 preys, have a kind of metamorphic 

 development. The embryonic fish has 

 no amnion, and the allantois, which is 

 represented by the urinary bladder, is 

 always rudimentary. Most fish are 

 covered by scales, which are formed 

 by the dermal or under layer of the 

 skin. The vertebrae are always bi- 

 concave or amphicoslous, the concav- 

 ities being filled in with notochordal matter. In the cartilag- 

 inous fishes, such as the sharks, we can see the vertebrfe being 

 drawn into the skull to take part in its formation. As, with 

 one or two exceptions, fishes are aquatic creatures, and as they 

 are of no importance agriculturally (except as manure ^), we may 

 thus summarily dismiss them. 



1 Fish are often used as manure in hop-gardens, &c., in httoral regions 

 when there has been a large glut on the market. 



Z 



Fig. 190.— DiAOHAM OF thi! Cir- 

 culation IN riSHE,S. 



a, Auricle ; v, ventricle ; m, 

 biilbus arteriosus ; n, brancliial 

 artery ; 6, vessels in gills ; c, 

 aorta. (Nicholson.) 



